This invention relates generally to improved method and apparatus for stabilizing the operation of an adjustable speed a-c electric motor that is driven by static electric power conversion apparatus. More particularly, the invention relates to an improved regulating scheme for stabilizing the operation of a current fed induction motor drive system, and it is also applicable to voltage fed induction motors and to drive systems employing synchronous or synchronous-reluctance motors.
In many applications of adjustable speed electric motors, alternating current induction motors are preferred to other kinds because of their relative simplicity, compactness, ruggedness, ease of maintenance, light weight, and low cost. Such a motor typically comprises a squirrel cage or wound rotor that is mounted in a stator having windings connected to a suitable source of excitation. The rotor is either rotatable (as in a round motor) or disposed for linear motion relative to the stator (as in a linear motor). In either case, when the stator windings are excited the magnetic flux across the stator-rotor air gap of the motor and the current induced in the rotor interact to produce an electromagnetic force (torque) tending to move the rotor relative to the stator. The amount of torque developed by the motor is often expressed in terms of the magnitude of the air gap flux and the slip frequency between stator and rotor. The effective slip frequency by definition is the difference between the frequency of the flux wave in the air gaps and the equivalent electrical frequency at which the motor shaft is rotating (i.e., motor speed). Where such a motor is required to run at variable speeds with variable loads and in both forward and reverse directions, as in the case of traction motors for electrically propelled vehicles, the stator windings are advantageously supplied with polyphase a-c power which is so conditioned that the frequency as well as the amplitude of the stator excitation are adjustable as desired and the phase sequence is reversible.
In current fed induction motor drive systems, the amplitude and frequency of the alternating current that excites the stator windings of the motor are controlled, in contrast to systems of the voltage fed type wherein the controlled quantities are the amplitude and frequency of the fundamental alternating voltage applied to the stator terminals. In either case, the source of excitation advantageously comprises controllable electric power conversion apparatus which is energized in turn by an available supply of direct current (d-c) or alternating current (a-c) power. Typically the conversion apparatus includes a controllable d-c power supply comprising either a d-c/d-c chopper whose input terminals are adapted to be coupled to an uncontrolled d-c source, a phase controlled rectifier circuit adapted to be coupled to fixed frequency a-c mains, or an uncontrolled rectifier adapted to be coupled to a variable alternating voltage source, and an inverter having a pair of d-c terminals coupled to the output terminals of the d-c power supply and a set of a-c terminals to which the stator windings of the induction motor are connected. The inverter is formed by a plurality of controllable electric valves or switching elements (e.g., thyristors) of the kind having the ability to hold off forward voltage until turned "on" in response to a suitable firing or gate signal; once a valve is triggered or fired by its gate signal, it switches from a blocking or non-conducting state to a forward conducting state in which it can freely conduct load current until this current is subsequently extinguished by the action of associated commutating means. In a current fed induction motor drive systems, the d-c link between the inverter and the controllable d-c power supply ordinarily includes a current smoothing filter. In such a system the amplitude of alternating current supplied to the motor can be regulated or controlled as desired by adjusting the average magnitude of voltage impressed on the d-c link, while the frequency of this current is controlled by appropriately varying the switching frequency of the controllable electric valves in the inverter.
Persons working in the art of adjustable speed a-c motor drives are continually seeking new ways to increase the accuracy, reduce the response time, and improve the reliability of control and regulating systems for such drives. See for example U.S. Pat. No. 3,700,986 - Cushman and Clark and 3,824,437 - Blaschke. In U.S. published patent application B511,886, (now U.S. Pat. No. 3,989,991), Brennan and Abbandanti review the prior art approach of maintaining a desired level of flux in the stator-rotor gap of a current fed motor by interdependently programming the motor slip frequency and the stator current magnitude, and they disclose an advantageous method of calculating slip frequency from terminal voltage and current without using tachometer feedback, thereby eliminating the expense and mechanical problems associated with a tachometer. Slip frequency control forces the inverter switching frequency to change in response to rotor speed. Although this has a stabilizing effect, it is per se not capable of ensuring stable operation of the system under all possible conditions. German patent DT25 16 247 discloses an alternative control strategy wherein the stator excitation frequency is adjusted so as to maintain a fixed phase angle between stator voltage and current (i.e., a constant power factor). Implementing either of these prior art schemes requires accurate knowledge of motor parameters. Since motor parameters tend to change with stator current, air gap flux, rotor frequency, and temperature, such schemes are difficult to operate over wide ranges of speed and load, and their accuracy is particularly poor at low speeds where the voltage drop across the stator impedance becomes the dominant portion of terminal voltage.
None of the prior art that we are presently aware of is optimum in terms of versatility of the controls and stability of the motor under a full range of dynamic conditions that can be encountered in practice, including high-speed motoring with maximum voltage being supplied by the controllable d-c power supply, and a zero-speed transition between braking and motoring modes of operation.
When a current fed induction motor drive system experiences a sudden alternation of the commanded or actual load, it is subject to an oscillatory effect similar to that of a synchronous motor under the same circumstances. In a synchronous motor damping of rotor swings is obtained from short-circuited rotor windings which generate transient voltages which in turn drawn transient current from the power supply to damp the oscillation, but in the case of an induction motor supplied by a controlled current inverter, the required damping current does not inherently flow from the excitation source. Recognizing this problem, Rettig in his U.S. Pat. No. 3,962,614 proposed adding to the slip regulating loop suitable means for advancing or retarding the inverter firing pulses as a preselected function of desired torque/slip, which function is selected so as to anticipate, for any new values of torque and slip frequency, the proper phase displacement of stator current with respect to its flux producing component. This suggested solution to the hunting problem does not inherently result in stable operation which, in Rettig's current fed motor drive, is realized by virtue of regulation of stator current.
As disclosed in the above-referred Rettig patent, the magnitude of stator current is regulated as a predetermined non-linear function of the torque command signal, which function is selected so that stator current will have the proper relation to slip frequency to maintain a substantially constant level of air gap flux in the motor regardless of its speed. With a relatively high slip frequency set by a correspondingly high torque command signal, and with constant flux, the terminal voltage on the stator of the motor tends to increase with increasing speed (flux being generally proportional to the amplitude-to-frequency ratio of the stator voltage), thus necessitating a proportionate increase in the voltage impressed on the d-c link of the power conversion apparatus. Eventually a speed can be reached at which the impressed voltage is maximum, whereupon the current regulating loop becomes saturated. Since current magnitude regulation is the stabilizing influence in this prior art control scheme, the system becomes unstable when the saturation point is reached. A possible solution to this instability problem is to limit the maximum stator voltage to a level appreciably lower than the maximum voltage capability of the front end of the conversion apparatus. A scheme for doing this is disclosed in Rettig's earlier U.S. Pat. No. 3,769,564 wherein the slip frequency of the motor is increased proportionately with speed if the stator voltage tends to exceed a predetermined limit. In this way, motor flux can vary inversely with speed above the base speed at which the voltage limit is reached, and consequently a relatively constant horsepower mode of operation is obtained. The difficulty with this solution to the stability problem is that it prevents the full power capabilities of the source and of the converter from being realized, and it is subject to misoperation in the event of short-term reductions in the supply voltage. Another disadvantage of stabilizing by current regulation is that a separate chopper or phase controlled rectifier circuit needs to be employed for each controlled current inverter/motor set.
In the prior art pertaining to adjustable speed synchronous motor drive systems, it has heretofore been suggested that controlling the power angle of a synchronous motor will stabilize its operation without damper windings. See Slemon, Forsythe and Dewan, "Controlled-Power-Angle Synchronous Motor Inverter Drive System" IEEE Trans. Industry Application, Volume IA-9, pp. 216-19 (March/April 1973). In the system disclosed by those authors a rotor position sensor coupled to the shaft of the synchronous motor monitors the angular position of the rotor, which information is used as a datum, and commands cyclic firing of the inverter valves in synchronism with rotor speed and so timed with respect to the datum as to result in a desired phase displacement .delta. between stator voltage and rotor current. In other words, the zero crossings of the alternating voltage applied to the stator windings of the synchronous motor are determined by the angular position of the rotor. Such a control system requires a mechanical position sensor, it suffers in accuracy, particularly at low speeds, because it neglects stator impedance, and in any event it cannot be feasibly implemented in induction motor drives.
Other problems are introduced in prior art adjustable speed a-c motor drives by inverter commutation. One type of commutation that is advantageously employed in current fed induction motor drive systems is known as auto-sequential commutation, and inherent in its operation is a time delay between the firing of an electric valve in each phase of the inverter and the instant of actual current transfer in the corresponding phase of the stator windings. Ripple currents introduce additional random delays in the commutation time. Such delays can cause difficulties in controlling the inverter and the motor.
Most of the shortcomings of the prior art can be avoided by using the improvement that is the subject matter of co-pending U.S. patent application Ser. No. 605,848 filed on Aug. 19, 1975, for J. D. D'Atre and A. B. Plunkett and assigned to the General Electric Company (now Pat. No. 4,044,285). That application teaches stabilizing a current fed induction motor drive system by controlling the frequency of the stator excitation current in a manner that regulates to a desired value the actual flux across the stator-rotor gap of the motor. For superior transient response, the excitation frequency control signal derived by the flux regulating loop is compensated by frequency sensitive means such as a phase lock-loop converter responsive to the frequency of the motor flux. See also co-pending U.S. patent application Ser. No. 605,847 (now U.S. Pat. No. 4,044,284) filed concurrently with Ser. No. 605,848 for the same inventors and assignee.